(1) Field of the Invention
The invention relates to the fabrication of semiconductor integrated circuit devices, and more specifically to a method of avoiding oxide residue after wolfram plug CMP oxide buffing.
(2) Description of the Prior Art
The invention relates to the field of Chemical Mechanical Polishing (CMP) of a semiconductor surface that contains wolfram plugs. More particularly, the present invention relates to methods for chemical mechanical polishing of substrates that contain wolfram plugs, such as semiconductor substrates, on a rotating polishing pad in the presence of a chemically and/or physically abrasive slurry, and providing fresh supply of slurry onto the surface of the substrate which is mounted on the polishing pad while the substrate is being polished.
U.S. Pat. No. 5,688,360 teaches that, during VLSI fabrication, meticulously clean silicon wafers are critical for obtaining high yields and suitable performance characteristics of semiconductor devices. Removal of impurities from the wafer surface is important because impurities may diffuse into the semiconductor substrate during subsequent high-temperature processing, altering the substrate bulk and surface properties. Some impurities are donor or acceptor dopants, which directly affect device performance characteristics. Other impurities cause surface or bulk defects such as traps, stacking faults or dislocations. Surface contaminants such as organic matter, oil or grease lead to poor film adhesion. The various types of impurities and contaminants must be removed by careful cleaning, such as chemical or ultrasonic cleaning at initiation of silicon processing and in various appropriate steps during processing.
U.S. Pat. No. 5,688,360 further teaches that silicon processing typically begins with a cleaning step involving wafer scrubbing to remove loose particulate contaminants. Particulates are bits of matter present on a wafer surface that have easily definable boundaries such as various dusts (atmospheric, silicon and quartz), lint, photoresist chunks and bacteria. Particulates are generally removed using a process herein called a cleaning process. Material that is too small to be measurable is herein referenced as `material`, which is generally removed using a polishing process.
Subsequent to a cleaning process, treatment with organic solvents, such as trichloroethylene, acetone, p-xylene, methanol and ethanol, is performed to remove organic impurities such as hydrocarbons and greases, which remain from a prior wafer-grinding process. A final cleaning includes treatment with several various inorganic chemicals to remove heavy metals, for example. These inorganic chemical mixtures are strong oxidants, which form a thin oxide layer at the wafer surface. This oxide layer is stripped, removing impurities absorbed into the oxide layer.
Chemical cleaning for removing chemically bonded film from wafer surface is one step in a cleaning process. Conventional chemical cleaning includes a series of acid and rinse baths.
For a typical CMP procedure, a polishing pad is pressed under high pressure against the wafer that is being polished. The polishing pad can be made of a synthetic material, polishing slurry is used to enhance and activate the polishing action. The polishing slurry typically contains pH-balanced chemicals, such as sodium hydroxide, and silicon dioxide particles. The fixture in which the wafer is mounted during the polishing operation rotates whereby the polishing pads is in direct physical contact with the wafer that is being polished. This direct contact combined with the frictional interaction between the wafer and the polishing pad create an abrasive action on the surface of the wafer that is being polished while the pH of the polishing slurry controls the chemical reactions of the chemicals that are contained in the insulating layer of the wafer. The polishing of the wafer is accomplished when the silicon dioxide particles abrade away the oxidized chemicals.
One factor, which contributes to the unpredictability and non-uniformity of the polishing rate of the CMP process, is the non-homogeneous replenishment of slurry at the surface of the substrate and the polishing pad. The slurry is primarily used to enhance the rate at which selected materials are removed from the substrate surface. As a fixed volume of slurry in contact with the substrate reacts with the selected materials on the surface of the substrate, this fixed volume of slurry becomes less reactive and the polishing enhancing characteristics of that fixed volume of slurry is significantly reduced. One approach to overcoming this problem is to continuously provide fresh slurry onto the polishing pad.
Specifically, applying the CMP process to Intra Level Dielectric (ILD) and Inter Metal Dielectric (IMD) that are used for the manufacturing of semiconductor wafers, surface imperfections (micro-scratch) typically present a problem. Imperfections caused by micro-scratches in the ILD and IMD can range from 100 to 1000 EA for 200 mm. wafers, where an imperfection typically has a depth from 500 to 900 A.sup.0 and a width of from 1000 to 3000 A.sup.0. As part of the polishing process of the ILD and IMD, a tungsten film is deposited; the surface imperfections will be filled with tungsten during this deposition. For devices within the semiconductor wafer with a dimension of 0.35 um. or larger, an etching process is used where the tungsten that has entered the imperfections within the wafer surface can be removed. For the larger size devices within the semiconductor wafer there is therefore no negative impact on the yield of these devices. For device sizes within the semiconductor wafer of 0.25 um or less, the indicated procedure of etching the tungsten layer is no longer effective. This results in relative large imperfections within the surface of the wafer, large with respect to the size of the semiconductor devices. These imperfections will cause shorts between the metal lines within the devices while the imperfections also have a severe negative impact on device yield and device reliability.
As part of the CMP process, oxide slurry can also be used. P typical CMP process uses only one polishing pad, this pad being a, hard polishing pad, for instance an IS 1000 pad. In polishing a semiconductor surface wherein wolfram plugs have been created as part of the damascene process, it is found that the plug protrudes from the semiconductor surface by about 1000 Angstrom, this protrusion will have an impact on the polishing process since the plug protrusion interrupts the constant and uniform contact between the polishing pad and the surface of the semiconductor wafer. From this it is readily apparent that oxide slurry will not be uniformly removed from the immediate vicinity of the wolfram plug and that a body of oxide slurry will accumulate near the plug. A particle count of in excess of 5000 EA has been observed in such an environment. It is the objective of the present invention to reduce this particle count in the immediate vicinity of the wolfram plug.
FIGS. 1 through 5 show the results obtained in Prior Art polishing of a semiconductor surface that contains wolfram plugs. FIG. 1 shows wolfram plugs 12 and 14 that have been created in a semiconductor substrate 10. The surface of layer 16 is polished, the results of this polishing process are shown in FIG. 2. Plugs 20 and 22 (FIG. 2) show the formation of keyholes 18, these keyholes result in poor planarization of the semiconductor surface. To remedy this poor planarization, the top of the wolfram plugs 20 and 22 is removed. This is done by first using film 30 of oxide slurry 30, FIG. 3, over the surface of the substrate 10. This oxide slurry, also called Fuse film, is typically between 1000 and 2000 K-Angstrom thick. FIG. 4 shows the results after the process of polishing using the oxide slurry has been completed whereby at the same time the top layer of the surface of the semiconductor is lowered, a total of between about 2K and 4K Angstrom is removed. FIG. 4 shows how the tops of plugs 20 and 22 protrude through the surface of substrate 10; the removal of the Fuse film uses NH.sub.4 OH and DIW for the removal process. FIG. 4 shows the results of the ideal case of the Fuse film removal, that is the case where no extraneous or loose oxide particles remain after the removal of the Fuse film. FIG. 5 shows the results of an actual case of Fuse film removal, it demonstrates that oxide slurry remains and has accumulated on top of the wolfram plugs, accumulations 24 and 26, and in the immediate vicinity of the wolfram plug, accumulation 28. For the Prior Art case as shown in FIG. 5, an oxide film of about 1 kAngstrom thickness will form on the surface of the wolfram plugs, this in addition to the indicated accumulation of the oxide slurries of 24, 26 and 28. In the case that is illustrated by FIG. 5, a defect count in the area of the wolfram plugs in excess of 5000 EA is consistently observed.
U.S. Pat. No. 5,244,534 (Yu et al.) shows a 2 step CMP process for W plugs. Yu appears to shows steps (1) and (2) of the invention. This is close to the invention.
U.S. Pat. No. 5,786,275 (Kubo) teaches a 2-step W CMP process. Kubo changes polishing conditions such as speed and slurry composition. Kubo does not appear to show the invention's (3).
U.S. Pat. No. 5,816,891 (Woo) teaches a CMP oxide and metal process/tool that has multiple pads and slurries.
U.S. Pat. No. 5,726,1099 (Jaso) shows a W CMP using ammonium Persulfate slurry.
U.S. Pat. No. 5,314,843 (Yu et al.) teaches a W CMP process.
U.S. Pat. No. 5,759,917 (Grover) shows an oxide CMP slurry.